For thermal stencil sheets, there have been conventionally used laminates of porous thin tissue paper and film of thermoplastic resins such as polyester. For a film used for such purpose, following properties are required.
(1) To have good sensitivity to perforation or pierceability (stencil formation). That is, the film should melt with a small amount of heat energy and have a sufficient thermal constriction property to form pierced holes of suitable sizes so that clear image is created in printing.
(2) To have strength and modulus of elasticity sufficient to stand working during the lamination with thin tissue paper and printing.
(3) To be able to give clear tone gradation. That is, when the film is used for stencil sheets, a film which may melt at the vicinities of holes to be formed by thermal piercing is not suitable because such a film does not give clear tone gradation in printing. The film should have thermal pierceability which makes clear difference between the points pierced and not pierced.
In addition to the above requirements, the film should have good productivity in manufacturing thereof. Specifically the film should have good drawability free from rupture and other troubles. It should have good slittability and good windability free from wrinkling and irregular winding when it is wound up.
There have been proposed use for such purpose of a biaxially stretched thermoplastic resin film, whose printing property is improved by specifying thermal characteristics thereof (Laying-Open Patent Application No. 62-149496) or by specifying thermal constriction properties thereof (Laying-Open Patent Publication No. 62-282983). However, they do not satisfy all the above requirements.
We conducted an intensive study to solve the problems and we found that a biaxially oriented polyester film which contains fine particles having a specified particular melting point and thermal constriction stress is suitable for thermal stencil sheets and thus completed the present invention.